1. Field of the Invention
The present invention relates to a process for preparing an ethylenic polymer composition and, more particularly, to a process for preparing an ethylenic composition having excellent physical properties such as high impact properties, environmental stress cracking resistance (ESCR), pinch-off fusing characteristics, and so on.
2. Description of Related Art
As a process for preparing blow-molding polyethylene having a wide range of molecular weight distribution, is there a known two-stage polymerization method. Polyethylene prepared by the two-stage polymerization method are excellent in a balance between toughness and environmental stress cracking resistance (ESCR), compared with polyethylene prepared by one-stage polymerization method, however, the former has the drawbacks that a tolerance for a pinch-off shape of the mold is narrow because a fusing strength of the pinch-off portion of a hollow molding is small, and a ratio of defective products is high and that die swell is small.
In order to improve the drawbacks of the conventional two-stage polymerization method, Japanese Patent Examined Publication No. 10,724/1984 proposes a three-stage polymerization method. This three-stage polymerization method can improve the die swell for the resulting polyethylene to a sufficient extent, however, improvements in the pinch-off fusing characteristics are not sufficient yet. Further, the three-stage polymerization method presents the disadvantages that it uses three reaction vessels so that control of polymerization is so complicated and costs of equipment become expensive.
Japanese Patent Unexamined Publication No. 85,690/1985 and Japanese Patent Examined Publication No. 27,677/1987 propose processes for preparing alpha-olefin polymers using one polymerization vessel in the presence of a catalyst obtainable from an organometallic compound, a magnesium compound, a titanium compound and an aluminium compound.
As a result of comparative studies, it has been found that this polymerization process is difficult to provide a polymer having physical properties required for blow molding, such as high impact properties, ESCR, pinch-off fusing characteristics, and so on.